It is known from internal combustion engines with catalytic converter to arrange one lambda probe each in front of and behind the catalytic converter. The front one measures an actual lambda value front and the rear one an actual lambda value rear. The actual lambda value front is subtracted from the lambda control desired value to which controlling is to be effected. The system deviation formed in this manner is converted by a means for lambda control into a manipulated variable which is dimensioned such that the system deviation is to be eliminated by the manipulated variable. The actual lambda value rear is used for monitoring the catalytic converter activity.
It is known that the actual lambda value rear fluctuates less than the actual lambda value front and that it provides more accurate information on the actual lambda value. This is because the lambda value measured by a lambda probe depends not only on the oxygen content of the measured mixture but also on the content of unburnt hydrocarbons. In the catalytic converter, a residual combustion and an equalization of fluctuations occur, as a result of which the rear lambda probe can very accurately determine the actual lambda value of the air/fuel mixture supplied to the internal combustion engine.
Because of the high accuracy of the actual lambda value rear, it is desirable to form the system deviation with the aid of this actual value. However, this cannot lead to practical results because a very large dead time passes between the preparation of an air/fuel volume and the time at which this volume reaches the rear lambda probe, now as burnt mixture. This makes it impossible to provide meaningful control. However, it would be possible to correct a manipulated variable formed by a means for lambda control with the aid of the actual lambda value rear with a manipulated variable which is formed by a second faster means for lambda control with the aid of the actual lambda value front. However, such an arrangement would result in stability problems.
U.S. Pat. No. 4,251,989 discloses a method which helps to prevent the above-mentioned stability problems by using the front probe as a control sensor while at the same time the advantages of the rear probe with respect to lower signal fluctuations are included in the control. The method utilizes the condition that a falsified signal of the probe ahead of the catalytic converter and used for control leads to asymmetry in the output signal of the probe mounted to the rear of the catalytic converter. This dissymmetry is detected via an integrator and is used for changing a comparative threshold which is compared to the output signal of the control probe and which output signal is otherwise influenced. The mixture formation can then be influenced in an ideal manner via units connected downstream so that this change leads to a compensation of the mixture shift caused by the false signal of the first probe.
However, one of the disadvantages of this method is that an oscillating output signal (FIG. 4D, two-step action) of the rear probe is absolutely necessary. In this way, the method is especially not applicable when the control of the lambda value is to a value unequal to zero.